The present invention relates to an economical power supply topology, which provides 100 percent of the required power to residential, commercial or industrial consumers, and that, further, protects consumers from power surges, dips, and outages; and, more particularly, to a on-site, grid-linked distributed electricity generation power supply topology that draws power for normal operation from fuel cells and/or other alternative energy sources, relying, on power from a public utility grid, which is coupled to an inverter through the direct current bus rather than in parallel with the inverter, to meet abnormal or anomalous peak power demand.
Foreseeable energy shortages from conventional electrical power sources and global concerns about the environment have sparked greater interest in alternative energy sources. These alternative sources include fuel cells, which produce electrical power by electrochemical reactions, and other means that produce power from wind or wave action, photovoltaic (solar) cells, micro-turbines et cetera. Unlike fossil fuels, renewable energy sources, such as wind power, wave power and solar power, are inexhaustible and environmentally friendly. However, power generated by wind, waves or the sun is highly dependent on weather and meteorological conditions; thus, subject to interruption. Fuel cells are relatively clean and efficient; however, they are limited to a design load and have a relatively slow response time. Thus, fuel cells cannot respond immediately to sharp increases in demand. Batteries, which have an immediate response time, store rather than produce energy hence are only good until the battery has drained. Moreover, battery cost is directly proportional to the stored energy needed. In the existing application, batteries are used to provide peak power, and a fuel cell is used to provide the continuous power, as well as to keep the battery charged.
Two methods exist for providing distributed power, which is defined as modular electrical generation or storage at or very close to the point of use, to consumers from alternative energy sources, such as fuel cells, batteries, wind turbines, etc. The first means is by grid independent architecture, which implies that distributed power delivered comes completely from the output power of an inverter, which converts energy from at least one fuel cell, battery, and/or other alternative power source into alternating current. Inherent in grid independent architecture is a need for sufficient distributed power to supply maximum, or peak, current demand. Hence, to be effective, the sum of the power capabilities of all of the connected energy sources, including fuel cells, batteries or other alternative energy sources must be designed to provide peak power on a worst-case basis, even though peak power demand may only occur, if at all, a few times a year and, even then, relatively briefly. Also, energy generating sources must be sized for the maximum continuous load the system would ever deliverxe2x80x94an expensive proposition considering the low frequency of such an occurrence. Typically, what is done is to size only the battery energy conversion equipment for this high power case, which works for very short periods of time at high load, within the limits of battery energy storage. Consequently, grid independent architecture suffers from over design and is inherently less economical and less cost efficient than the second means, which is to say grid parallel architecture. Grid independent architecture, further, cannot satisfy demand in excess of the demand for which it was designed. So, abnormal or anomalous demands that exceed the design peak demand may overtax a grid independent architecture.
The alternative to grid independent architecture is grid parallel architecture. Grid parallel architecture delivers distributed power from a fuel cell, battery or other alternative energy source as well as power from a public utility grid. The redundancy of the two power sources, which are parallel systems, provides the ability for the power supply to deliver a constant level of power at its output. This is most beneficial when the source of power is unpredictable, such as solar. In the case of solar, it is a rare event when the load power matches available power. In this case, if the distributed power source cannot provide sufficient power to meet demand, power from the utility grid makes up the difference. Hence, alternative energy sources do not have to be designed for a worst case scenario. Furthermore, the utility grid provides redundancy and peak capability to the alternative energy source. Hence, there is a cost saving in not providing redundant fuel cell, battery, and/or other alternative energy source, which would only operate during abnormal or peak demand.
Another advantage of a grid parallel topology is that utility grid absorbs surplus power generated by the distributed power source, which surplus power is available to help meet peak demands elsewhere on the public utility""s network.
However, the consequence of grid parallel topology is a requirement for inter-connection between the inverter of the distributed power source and the utility grid. A recent study by the National Renewable Energy Laboratory entitled xe2x80x9cMaking Connections: Case Studies of Interconnection Barriers and their Impact of Distributed Power Projectsxe2x80x9d, which is incorporated herein by reference, highlighted the technical, business-practice and regulatory barriers to the interconnection of an alternative energy source distributed power source and a public utility grid. Technical barriers to interconnection include, without limitation, personnel safety, power quality, operation of the local distribution system, and compatibility with the utility grid and grid operation. Business-practice barriers include, without limitation, lengthy contractual and procedural requirements, application fees, insurance requirements, and operational requirements, all of which consume time and increase the cost of installing a distributed power alternative energy source. Finally, regulatory barriers include, without limitation, absolute prohibition, disincentives in the form of discounted energy from the public utility, special fees and tariffs, and environmental permitting. There are no national, or federally mandated, standards for the application process; hence, each public utility may have unique fees, rules, approval processes, and specifications for permitting power generation into a utility grid, potentially requiring multiple applications to a myriad of public utilities. Additionally, as provided above, some public utilities do not compensate or unfairly under-compensate consumers for surplus power supplied into the utility grid.
Therefore, a need exists for a distributed power-generating source that benefits from many of the advantages offered by both a grid independent and a grid parallel topology. Such a solution should reduce the cost and delay associated with regulatory, contractual and procedural requirements while simultaneously providing power more reliably, by virtue of the ability to draw power from the grid, and more efficiently and more cost effectively, by sizing the alternative energy source for only normal demand. In addition it is desirable for the owner of the distributed generator to have a feeling of independence from the utility grid.
In this setting, it would be desirable to provide a grid-linked power supply (xe2x80x9cGLPSxe2x80x9d), comprising a distributed power source comprising fuel cells, and/or other alternative energy sources, that is intermediate to grid independent and grid parallel architectures. Indeed, it would be particularly desirable to provide a GLPS, wherein the alternative energy source provides continuous demand load, relying on clean, efficient, and economical fuel cells, and/or other alternative energy sources for normal power demands and, further, on a public utility grid for peak, abnormal or anomalous power requirements. Furthermore, it would be desirable to provide a GLPS that does not require public utility approval or other regulatory permitting generally associated with interconnection of a distributed power source with a public utility grid.
Thus, the present invention provides a grid-linked power supply comprising an inverter, at least one distributed energy source, e.g., a fuel cell, to meet normal, non-peak power demand, a connection to a public utility grid to meet peak, abnormal or anomalous power demand requirements, and a converter for regulating delivery of power from the distributed energy source or the public utility grid.
Preferred embodiments of the present invention provide a grid-linked power supply that more reliably provides 100 percent of required power from the distributed energy source. The distributed energy source can include energy storage (e.g., battery power) augmentation for additional peak power capability. Thus, the user does not rely on the public utility grid for normal operations.
Certain embodiments of the present invention provide a grid-linked power supply that allows for periodic maintenance of the fuel cell or other alternative energy source without interrupting power delivery. In preferred embodiments, the customer can prioritize usage of the grid versus usage of the energy storage device to provide a range of choice between battery life and grid independence. For example, the customer can use the utility grid before using a battery, resulting in longer battery life. Alternatively, the customer can use the battery before using the utility grid, providing the owner with the lowest possible electric utility bill, and greater independence from the grid.
Preferred embodiments of the invention provide a grid-linked power supply that is economical and efficient.
Certain embodiments of the present invention provide a grid-linked power supply topology, wherein the primary power producing means comprises at least one fuel cell, and/or other alternative energy source, which singly or in combination with each other or with a battery or other energy storage device such as a flywheel or ultra-capacitor, generate sufficient continuous power for normal demands of residential, commercial or industrial consumers. Fuel cells and/or other alternative energy sources decrease the emission of pollutants and, moreover, public utility power generating requirements. Furthermore, use of fuel cells and/or other alternative energy sources as a primary power producing means protects consumers against damage or loss resulting from power surges, dips, and/or outages, making the topology more reliable than the utility grid alone. The grid-linked power supply topology comprises further a connection to a public utility grid, which is connected to the direct current bus of the inverter rather than in parallel with the output of the inverter, to provide auxiliary power during infrequent, and unpredictable periods of peak, anomalous or abnormal demand. Connecting the utility grid to the direct current bus of the inverter, moreover, ensures that surplus power generated by the fuel cells, batteries, and/or other alternative energy sources is not generated into the utility grid, eliminating the potential for grid parallel operation and thus the costly and time-consuming grid interconnection application and approval processes. The GLPS controls the switching between the fuel cell, battery, and/or alternative energy source and the utility grid by providing a DC/DC converter for each GLPS energy source. Each converter produces a limited current and each converter has distinct voltage set points and, additionally, the grid voltage is provided with yet another voltage set point.
Using a utility grid to provide peak, anomalous and abnormal demand power ensures that fuel cells, batteries, and/or other alternative power sources are more efficiently designed and sized for normal power demands, which is to say that the primary distributed energy sources do not have to be sized for the worst case condition. Therefore, the present invention can provide power more economically. Moreover, using a utility grid to provide peak power demand ensures that peak power can be delivered, if necessary, for a longer period of time or, ideally, continuously. Hence, the invention disclosed herein can be more reliable than a grid parallel or grid independent system. As an example, if one defines normal continuous power as 1x, and defines an extended peak power as 2xc3x97normal power for a long time ( greater than 5 sec.) and a short peak power as 4xc3x97normal power for a short time ( less than 5 sec.), the described present invention can provide extended peak operation. This capability can be enhanced with additional batteries and/or other energy storage elements, however such storage elements have limited energy storage and as such, cannot provide such power indefinitely. The present invention gives the systems integrator the ability to offer the tradeoff between shorter battery life and greater maintenance and reduced size and cost.
Other aspects and embodiments of the invention are discussed below. Moreover, additional advantages of the present invention will be apparent from the drawings and specifications that follow.